Dogs are
no longer treated as strict carnivores; they are now omnivores that eat a
variety of foods. Cats are different with metabolic processes of a strict
carnivore. Their requirements constrain the formulation of diets for cats
more so than for dogs. Cats' diets must be formulated differently from dogs'
diets.

Digestibility of Foods

Because
of the cat's anatomy and physiology it may seem that the digestibility of
many natural food ingredients, particularly those of vegetable origin,
should be lower in cats than it is in most other animals. This is based on
the cat's shorter small intestine, lack of cecum with its bacterial
population, and generally shorter transit time. Design of the feline
intestine is for a high-fat, high-protein diet that is, a high-energy,
low-bulk diet. Despite this, most commercial cat foods contain many
vegetable products, mainly cereal grains which are more incompletely
digested than animal products. These diets cause no problems due to
nutritional deficiency because essential amino acids, primarily taurine, and
fatty acids are added. In addition, the diets contain salt and mineral
contents for maintaining acid-base balance.

Unique Vitamin Requirements

Cats do
not have the ability to convert carotene to vitamin A. Lacking the enzyme
for that conversion makes it necessary for cats to have vitamin A in their
diet. Cats also do not have the capacity for converting tryptophan to the
niacin. Cats metabolize tryptophan too rapidly to other compounds. The diet
must provide niacin. Cats and dogs cannot manufacture vitamin D or its
precursor 7-dehydrocholesterol. Thus, the diet must provide vitamin D.

Protein and Amino Acid
Requirements

A cat's
diet must provide proteins and amino acids containing a source of essential
amino acids for which cats have an absolute requirement. Dietary proteins
must also provide a greater source of nitrogen than that needed by most
other animals. Cats do not conserve nitrogen as well as other animals. Their
enzyme activities for metabolizing amino acids are greater than in other
animals and that activity does not decrease when they eat a low protein
diet. Excess amino acid destruction continues, leaving insufficient amounts
for making protein. Feeding low protein diets is always wrong for cats.

Many
animals can survive on a low protein intake of 4% to 8% of the total dietary
calories. In contrast, cats need 18% to 20% of total calories as protein for
growth and 12% to 13% for adult maintenance. Thus the cat needs two to three
times more protein than most other animals under comparable circumstances.
The 18% to 20% of total calories for a growing kitten represents about 25%
of the dry weight of the diet. Commercial cat foods contain 30% to 35%
protein on a dry basis, which is an excess because cats poorly digest the
proteins in these diets. Diets formulated for dogs contain too little
protein for feeding cats.

Cats
cannot synthesize the essential amino acid citrulline that is low in any
food. Cats can convert arginine to citrulline, however, and that means that
feline diets must contain arginine to meet the need for citrulline. Cats fed
a diet lacking arginine develop hyperammonemia and show clinical signs of
illness within several hours. Ammonia accumulates because it it is not
converted to urea; arginine and citrulline are needed for that conversion.
Feline diets must contain arginine.

Cats
have only a limited ability to synthesize the essential amino acid taurine
from sulfur-containing amino acids. Therefore, a cat's diet must provide
taurine. It also helps for the diet to be rich in sulfur-containing amino
acids. Diets low in protein, and therefore sulfur-amino acids, are more
likely to induce taurine deficiency.

Taurine
is the most abundant free amino acid in the body. It is not incorporated
into body proteins. Its many important functions include being a precursor
for bile salts (both cats and dogs have an obligatory and continuous
requirement for taurine to make bile salts to replace bile salts lost
continuously in the feces). Taurine is also involved in growth and
maturation of nervous tissue, maintenance of integrity for the eyes’ rods
and cones, normal heart function, and female reproduction. Taurine is found
in all animal tissues but not in plant materials.

Since
taurine is free, not incorporated in proteins, in animal tissues, it readily
leaches out in water. Cooking meat in water and discarding the water can
greatly reduce its taurine content. Proteins from plants such as soybeans
and from animal products such as cottage cheese provide no taurine. These
foods also greatly reduce a cat's ability to maintain normal plasma taurine
concentrations. Canned diets require higher concentrations of taurine to
maintain normal levels than dry foods. No reason is known for this
difference other than the two diets have very different formulations.

Within the
last decade two diseases, dilated cardiomyopathy and central retinal
degeneration, appeared in cats fed commercial diets containing insufficient
taurine. (Surprisingly the foods' taurine concentrations were those recommended
by the NRC.) Taurine deficiency results in other important diseases. Only some
of these problems can be reversed with taurine supplementation. Since some
problems cannot be corrected, it is important to assure that the content of
taurine is adequate for any feline diet. Taurine deficiency does not appear in
cats living under natural conditions, catching their own food, or where the
animal is eating what nature designs a carnivore to eat, meat.

Cats have a
deficiency of the enzyme needed to synthesize arachidonic acid from linoleic
acid. Therefore, the diet's supply of unsaturated fatty acids needs to include
arachidonic acid.

Carbohydrate Metabolism

The natural
diet of a true carnivore, such as the cat, is normally very low in carbohydrate,
being mainly protein and fat. Blood glucose levels remain normal in carnivores
as well as in omnivores but not principally because of the carbohydrates they
eat. Carnivores convert amino acids and glycerol for glucose. A carnivore's
liver does not receive high amounts of glucose from a carbohydrate meal.
Therefore, the carnivore's liver is not prepared to store much glycogen made
from carbohydrates. Cats can maintain blood glucose levels on starvation diets
better than other starving animals usually fed high protein diets. Cats are also
able to store more glycogen in the liver than others when fed a high protein
diet.

Tolerance to Food Additives and Some
Unique Foods

Cats are
more sensitive to food additives than other animals. Cats are intolerant to the
common food preservative, benzoic acid. Benzoic acid occurs naturally in plants
but is not found in animal tissues. Cats have a limited ability to metabolize
the chemical to a form that the body can excrete. Aspirin is a form of benzoic
acid that cats excrete poorly.

Cat foods
can contain propylene glycol which can damage erythrocytes. The result is a
shorter life span that can lead to anemia. Relatively small amounts of onions
can poison cats. Documented cases appeared after eating onion soup and baby
foods containing onion as a flavoring agent. Cocoa and chocolate can poison dogs
and the toxic derivative, theobromine, can be lethal for cats.

Unknown Intolerances

Cats may
have other unique nutritional needs that are currently unknown. Feline
hyperthyroidism has been associated with feeding commercial canned cat food.
This disease may be related to cats' unique needs that are not being met.

Addendum 2010

The
cause of feline hyperthyroidism is still unexplained. Many articles on this
disease do not discuss any possible causes but some do indicate that feeding
commercial canned cat food is still the most likely possibility but no one has
ventured to investigate this as a cause.